Once this is done, you can execute the config file by opening the console and typing:

exec myconfigname.cfg

This will run whatever commands are in the file.

Since you can bind commands to keyboard keys, you can extend this to "exec" commands by pressing keys, for instance:

bind "KP_5" "exec myconfigname.cfg"

would make the 5 key on your keypad execute the config file "myconfigname.cfg". Since config files can rebind keys, you can get pretty fancy with turning things on and off - bind a key in each config file that cycles through your configs, for instance.

There's a lot of power in the console (and, by extension, config files) - there's a good tutorial on the Steam forums here which I referenced in making this answer.